Encoder Transmitte Channell Receive FIGURE 78.1 Basic communication link. Personal computers and workstations tend to be resources dedicated to a specific individual or project. Since the communications engineer working at his or her desk has control over the computing resource, lengthy simulations can be performed without interfering with the work of others. Over the past few years a number of software packages have been developed that allow complex communication systems to be simulated with relative ease [Shanmugan, 1988]. The best of these packages contains a wide variety of subsystem models as well as integrated graphics packages that allow waveforms, spectra, histograms, and performance characteristics to be displayed without leaving the simulation environment. For those motivated to generate their own simulation code, the widespread availability of high-quality C, Pascal, and FORTRAN compilers makes it possible for large application-specific simulation programs to be developed for personal computers and work- stations. When computing tools are both available and convenient to use, they will be employed in the day-to- day efforts of system analysts and designers e The purpose of this chapter is to provide a brief introduction to the subject of computer-aided design and analysis of communication systems. Since computer-aided design and analysis almost always involves some level of simulation, we focus our discussion on the important subject of the simulation of communication systems Computer simulations can, of course, never replace a skilled engineer, although they can be a tremendous help in both the design and analysis process. The most powerful simulation program cannot solve all the problems that arise, and the process of making trade-off decisions will always be based on experience. In ddition, evaluating and interpreting the results of a complex simulation require considerable skill and insight. While these remarks seem obvious, as computer-aided techniques become more powerful, one is tempted replace experience and insight with computing power. 78.2 The role of simulation The main purposes of simulation are to help us understand the operation of a complex communication system to determine acceptable or optimum parameters for implementation of a system, and to determine the per formance of a communication system. There are basically two types of systems in which communication engineers have interest: communication links and communication networks A communication link is usually a single source, a single user, and the components and channel between source and user. A typical link architecture is shown in Fig. 78. 1. The important performance parameter in a digital communication link is typically the reliability of the communication link as measured by the symbo or bit error rate(BER). In an analog communication link the performance parameter of interest is typically the signal-to-noise ratio (SNR)at the receiver input or the mean-square error of the receiver output. The simulation is usually performed to determine the effect of system parameters, such as filter bandwidths or code rate,or to determine the effect of environmental parameters, such as noise levels, noise statistics, or power ral o A communication network is a collection of communication links with many signal sources and many users. Computer simulation programs for networks often deal with problems of routing, flow and congestion control and the network delay. While this deals with the communication link, the reader is reminded that network simulation is also an important area of study. The simulation methodologies used for communication networks are different from those used on links because, in a communication link simulation, each waveform present in the system is sampled using a constant sampling frequency. In contrast, network simulations are event-driven, with the important events being such quantities as the time of arrival of a message e 2000 by CRC Press LLC© 2000 by CRC Press LLC Personal computers and workstations tend to be resources dedicated to a specific individual or project. Since the communications engineer working at his or her desk has control over the computing resource, lengthy simulations can be performed without interfering with the work of others. Over the past few years a number of software packages have been developed that allow complex communication systems to be simulated with relative ease [Shanmugan, 1988]. The best of these packages contains a wide variety of subsystem models as well as integrated graphics packages that allow waveforms, spectra, histograms, and performance characteristics to be displayed without leaving the simulation environment. For those motivated to generate their own simulation code, the widespread availability of high-quality C, Pascal, and FORTRAN compilers makes it possible for large application-specific simulation programs to be developed for personal computers and work￾stations. When computing tools are both available and convenient to use, they will be employed in the day-to￾day efforts of system analysts and designers. The purpose of this chapter is to provide a brief introduction to the subject of computer-aided design and analysis of communication systems. Since computer-aided design and analysis almost always involves some level of simulation, we focus our discussion on the important subject of the simulation of communication systems. Computer simulations can, of course, never replace a skilled engineer, although they can be a tremendous help in both the design and analysis process. The most powerful simulation program cannot solve all the problems that arise, and the process of making trade-off decisions will always be based on experience. In addition, evaluating and interpreting the results of a complex simulation require considerable skill and insight. While these remarks seem obvious, as computer-aided techniques become more powerful, one is tempted to replace experience and insight with computing power. 78.2 The Role of Simulation The main purposes of simulation are to help us understand the operation of a complex communication system, to determine acceptable or optimum parameters for implementation of a system, and to determine the per￾formance of a communication system. There are basically two types of systems in which communication engineers have interest: communication links and communication networks. A communication link is usually a single source, a single user, and the components and channel between source and user. A typical link architecture is shown in Fig. 78.1. The important performance parameter in a digital communication link is typically the reliability of the communication link as measured by the symbol or bit error rate (BER). In an analog communication link the performance parameter of interest is typically the signal-to-noise ratio (SNR) at the receiver input or the mean-square error of the receiver output. The simulation is usually performed to determine the effect of system parameters, such as filter bandwidths or code rate, or to determine the effect of environmental parameters, such as noise levels, noise statistics, or power spectral densities. A communication network is a collection of communication links with many signal sources and many users. Computer simulation programs for networks often deal with problems of routing, flow and congestion control, and the network delay. While this chapter deals with the communication link, the reader is reminded that network simulation is also an important area of study. The simulation methodologies used for communication networks are different from those used on links because, in a communication link simulation, each waveform present in the system is sampled using a constant sampling frequency. In contrast, network simulations are event-driven, with the important events being such quantities as the time of arrival of a message. FIGURE 78.1 Basic communication link. 8574/ch078/frame Page 1750 Wednesday, May 6, 1998 11:08 AM